An image sensor obtains pixel signals corresponding to irradiating light from a subject by a photoelectric conversion element. The image sensor then converts the pixel signals having analog values to digital data, and outputs the data as image data.
When a column analog-to-digital conversion circuit is used to digitize the pixel signals, the pixel signals for one row are read at the same time, and the pixel signals for one row are digitized.
In the digitization by the image sensor, correlated double sampling (CDS) processing, for example, is used as one method of removing variation components in pixels or a comparator. In the CDS processing, a reset voltage of the pixel is output, and a signal voltage corresponding to incident light is output from the pixel. Further, a difference between the signal voltage and the reset voltage is calculated, and an image signal free from the variation components is obtained.
However, if random noise is generated during the digitization or during the CDS processing, the noise is included in the output image.
For example, noise generated by switching in the analog-to-digital conversion circuit or noise of a bias voltage supplied to the circuit may cause the noise in the output image.